Unified communication is a technique that a number of equipment manufacturers such as CISCO®, AVAYA®, HP®, POLYCOM®, MICROSOFT® (UCS, now called LYNC®) and others use to make email, phone, and instant messaging (IM) clients compatible and easier to use. The goal of unified communication is to enable users to reach and collaborate more timely with remote and mobile co-workers, decision makers, and customers, which improves productivity and efficiency and results in better communication and faster decision-making. Unified communication creates the opportunity to experience these benefits through the integration of real-time communications services including: video & audio conferencing, scheduling, whiteboards, presence/IM, unified messaging, VoIP providing, P2P voice, and PSTN termination/origination.
Today, unified communications is a vibrant technology, yet it is mired in a fragmented ecosystem. The goal of a seamless company-to-company communications (inter-domain federation), as well as that within a company (intra-domain federation), from one vendor's equipment to another remains elusive. To fully realize the opportunity that exists for unified communication, inter-vendor interoperability must be addressed within the industry.
The challenges to interoperability are compounded by the unified communication industry emerging from the communications industry silos that have materialized over the last 50 years. The various unified communication vendors all have their historical roots in different aspects of communications (e.g., telephony, video, devices, etc.) and are struggling to remain relevant in the unified communication era where few vendors provide an end-to-end solution. Even those vendors whom offer a full suite of unified communication products, find that their customers have existing investments in a range of vendor equipment within their technology portfolios.
The various unified communication implementations from a number of vendors present similar functionality and user experiences yet the underlying technologies are diverse, supporting multiple protocols that include: XMPP; SIMPLE for IM/P; H.323, SIP, XMPP/Jingle for Voice & Video. Additionally, there are disparate protocols for Data Conferencing Multiple Codec's used for voice and video: e.g., G.711/729, H.263/264, etc. Finally, there are many proprietary media stack implementations addressing IP packet loss, jitter and latency in different ways.
Unified communications (UC) is the integration of real-time communication services such as instant messaging (chat), presence information, telephony (including IP telephony), video conferencing, call control and speech recognition with non-real-time communication services such as unified messaging (integrated voicemail, email, SMS and fax). UC is not a single product, but a set of products that provides a consistent unified user interface and user experience across multiple devices and media types.
UC also refers to a trend to offer business process integration, i.e., to simplify and integrate all forms of communications to optimize business processes and reduce the response time, manage flows, and eliminate device and media dependencies.
UC allows an individual to send a message on one medium and receive the same communication on another medium. For example, one can receive a voicemail message and choose to access it through email or a cell phone. If the sender is online according to the presence information and currently accepts calls, the response can be sent immediately through text chat or video call. Otherwise, it may be sent as a non real-time message that can be accessed through a variety of media.
UC is an evolving communications technology architecture which automates and unifies all forms of human and device communications in context, and with a common experience. Its purpose is to optimize business processes and enhance human communications by reducing latency, managing flows, and eliminating device and media dependencies.
Unified communications represents a concept where multiple modes of business communications can be seamlessly integrated. Unified communications is not a single product but rather a solution that consists of various elements, including, but not limited to, the following: call control and multimodal communications, presence, instant messaging, unified messaging, speech access and personal assistant, conferencing, collaboration tools, mobility, business process integration (BPI) and a software solution to enable business process integration.
The term “presence” is also a factor—knowing where one's intended recipients are and if they are available, in real time—and is, itself, a key component of unified communications. Simply put, unified communications integrates all the systems that a user might already be using and helps those systems work together in real time. For example, unified communications technology could allow a user to seamlessly collaborate with another person on a project, even if the two users are in separate locations. The user could quickly locate the necessary person by accessing an interactive directory, engage in a text messaging session, and then escalate the session to a voice call, or even a video call—all within minutes. In another example, an employee receives a call from a customer whom has a question. Unified communications could enable that worker to access a real-time list of available expert colleagues, then make a call that would reach the necessary person, enabling the employee to answer the customer faster, and eliminating rounds of back-and-forth emails and phone-tag.
The examples in the previous paragraph primarily describe “personal productivity” enhancements that tend to benefit the individual user. While such benefits can be important, enterprises are finding that they can achieve even greater impact by using unified communications capabilities to transform business processes. This is achieved by integrating UC functionality directly into the business applications using development tools provided by many of the suppliers. Instead of the individual user invoking the UC functionality to, for example, find an appropriate resource, the workflow or process application automatically identifies the resource at the point in the business activity where one is needed.
When used in this manner, the concept of presence often changes. Most people associate presence with instant messaging (IM “buddy lists”) the status of individuals is identified. But, in many business process applications, what is important is finding someone with a certain skill. In these environments, presence will identify available skills or capabilities.
This “business process” approach to integrating UC functionality can result in bottom line benefits that are an order of magnitude greater than those achievable by personal productivity methods alone.
Given the sophistication of unified communications technology, its uses are myriad for businesses. It enables users to know where their colleagues are physically located (say, their car or home office). They also have the ability to see which mode of communication the recipient prefers to use at any given time (perhaps their cell phone, or email, or instant messaging). A user could seamlessly set up a real-time collaboration on a document they are producing with a co-worker, or, in a retail setting, a worker might do a price-check on a product using a hand-held device and need to consult with a co-worker based on a customer inquiry. With unified communications, instant messaging and presence could be built into the price check application, and the problem could be resolved in moments.
SIP
The session initiation protocol (SIP) is an IETF-defined signaling protocol, widely used for controlling multimedia communication sessions such as voice and video calls over Internet protocol (IP). The protocol can be used for creating, modifying and terminating two-party (unicast) or multiparty (multicast) sessions consisting of one or several media streams. The modification can involve changing addresses or ports, inviting more participants, and adding or deleting media streams. Other feasible application examples include video conferencing, streaming multimedia distribution, instant messaging, presence information, file transfer and online games.
SIP was originally designed by Henning Schulzrinne and Mark Handley starting in 1996. The latest version of the specification is RFC 3261 from the IETF Network Working Group.
In November 2000, SIP was accepted as a 3GPP signaling protocol and permanent element of the IP Multimedia Subsystem (IMS) architecture for IP-based streaming multimedia services in cellular systems.
The SIP protocol is an application layer protocol designed to be independent of the underlying transport layer; it can run on transmission control protocol (TCP), user datagram protocol (UDP), or stream control transmission protocol (SCTP). It is a text-based protocol, incorporating many elements of the hypertext transfer protocol (HTTP) and the simple mail transfer protocol (SMTP).
SIP employs design elements similar to the HTTP request/response transaction model.
Each transaction consists of a client request that invokes a particular method or function on the server and at least one response. SIP reuses most of the header fields, encoding rules and status codes of HTTP, providing a readable text-based format.
SIP works in concert with several other protocols and is only involved in the signaling portion of a communication session. SIP clients typically use TCP or UDP on port numbers 5060 and/or 5061 to connect to SIP servers and other SIP endpoints. Port 5060 is commonly used for non-encrypted signaling traffic whereas port 5061 is typically used for traffic encrypted with transport layer security (TLS). SIP is primarily used in setting up and tearing down voice or video calls. It has also found applications in messaging applications, such as instant messaging, and event subscription and notification. There are a large number of SIP-related Internet engineering task force (IETF) documents that define behavior for such applications. The voice and video stream communications in SIP applications are carried over another application protocol, the real-time transport protocol (RTP). Parameters (port numbers, protocols, codecs) for these media streams are defined and negotiated using the session description protocol (SDP), which is transported in the SIP packet body.
A motivating goal for SIP was to provide a signaling and call setup protocol for IP-based communications that can support a superset of the call processing functions and features present in the public switched telephone network (PSTN). SIP by itself does not define these features; rather, its focus is call-setup and signaling. However, it was designed to enable the construction of functionalities of network elements designated proxy servers and user agents. These are features that permit familiar telephone-like operations: dialing a number, causing a phone to ring, hearing ringback tones or a busy signal. Implementation and terminology are different in the SIP world but to the end-user, the behavior is similar.
SIP-enabled telephony networks can also implement many of the more advanced call processing features present in signaling system 7 (SS7), though the two protocols themselves are very different. SS7 is a centralized protocol, characterized by a complex central network architecture and “dumb endpoints” (traditional telephone handsets). SIP is a peer-to-peer protocol, thus it requires only a simple (and thus scalable) core network with intelligence distributed to the network edge, embedded in endpoints (terminating devices built in either hardware or software). SIP features are implemented in the communicating endpoints (i.e., at the edge of the network) contrary to traditional SS7 features, which are implemented in the network.
Although several other VoIP signaling protocols exist, SIP is distinguished by its proponents for having roots in the IP community rather than the telecommunications industry. SIP has been standardized and governed primarily by the IETF, while other protocols, such as H.323, have traditionally been associated with the International Telecommunication Union (ITU).
The first proposed standard version (SIP 2.0) was defined by RFC 2543
SIP Network Elements
SIP user agent (UA) is a logical network end-point used to create or receive SIP messages and thereby manage a SIP session. A SIP UA can perform the role of a user agent client (UAC), which sends SIP requests, and the user agent server (UAS), which receives the requests and returns a SIP response. These roles of UAC and UAS only last for the duration of a SIP transaction.
A SIP phone is a SIP user agent that provides the traditional call functions of a telephone, such as dial, answer, reject, hold/unhold, and call transfer.
SIP phones may be implemented by dedicated hardware controlled by the phone application directly or through an embedded operating system (hardware SIP phone) or as a softphone, a software application that is installed on a personal computer or a mobile device, e.g., a personal digital assistant (PDA) or cell phone with IP connectivity. As vendors increasingly implement SIP as a standard telephony platform, often driven by 4G efforts, the distinction between hardware-based and software-based SIP phones is being blurred and SIP elements are implemented in the basic firmware functions of many IP-capable devices. Examples are devices from NOKIA® and RESEARCH IN MOTION®.
Each resource of a SIP network, such as a User Agent or a voicemail box, is identified by a uniform resource identifier (URI), based on the general standard syntax also used in Web services and email. A typical SIP URI is of the form: sip:username:password@host:port. The URI scheme used for SIP is sip: If secure transmission is required, the scheme sips: is used and SIP messages must be transported over Transport Layer Security (TLS).
In SIP, as in HTTP, the user agent may identify itself using a message header field “User-Agent,” containing a text description of the software/hardware/product involved. The User-Agent field is sent in request messages, which means that the receiving SIP server can see this information. SIP network elements sometimes store this information, and it can be useful in diagnosing SIP compatibility problems.
SIP also defines server network elements. Although two SIP endpoints can communicate without any intervening SIP infrastructure, which is why the protocol is described as peer-to-peer, this approach is often impractical for a public service.
RFC 3261 Defines these Server Elements:                A proxy server “is an intermediary entity that acts as both a server and a client for the purpose of making requests on behalf of other clients. A proxy server primarily plays the role of routing, which means its job is to ensure that a request is sent to another entity “closer” to the targeted user. Proxies are also useful for enforcing policy (for example, making sure a user is allowed to make a call). A proxy server interprets, and, if necessary, rewrites specific parts of a request message before forwarding it.”        “A registrar is a server that accepts REGISTER requests and places the information it receives in those requests into the location service for the domain it handles.”        “A redirect server is a user agent server that generates 3xx responses to requests it receives, directing the client to contact an alternate set of URIs. The redirect server allows SIP Proxy Servers to direct SIP session invitations to external domains.”        The RFC specifies: “It is an important concept that the distinction between types of SIP servers is logical, not physical.”        
Other SIP related network elements are Session border controllers (SBC), they serve as middle boxes between UA and SIP server for various types of functions, including network topology hiding, and assistance in NAT traversal.
Various types of gateways or bridges at the edge between a SIP network and other networks (as a phone network).
SIP Messages
SIP is a text-based protocol with syntax similar to that of HTTP. There are two different types of SIP messages: requests and responses. The first line of a request has a method, defining the nature of the request, and a Request-URI, indicating where the request should be sent.
The first line of a response has a response code.
For SIP requests, RFC 3261 defines the following methods:                REGISTER: Used by a UA to indicate its current IP address and the URLs for which it would like to receive calls.        INVITE: Used to establish a media session between user agents.        ACK: Confirms reliable message exchanges.        CANCEL: Terminates a pending request.        BYE: Terminates a session between two users in a conference.        OPTIONS: Requests information about the capabilities of a caller, without setting up a call.        
The SIP response types defined in RFC 3261 fall in one of the following categories:                Provisional (1xx): Request received and being processed.        Success (2xx): The action was successfully received, understood, and accepted.        Redirection (3xx): Further action needs to be taken (typically by sender) to complete the request.        Client Error (4xx): The request contains bad syntax or cannot be fulfilled at the server.        Server Error (5xx): The server failed to fulfill an apparently valid request.        Global Failure (6xx): The request cannot be fulfilled at any server.SIP Transactions        
SIP makes use of transactions to control the exchanges between participants and deliver messages reliably. The transactions maintain an internal state and make use of timers. Client Transactions send requests and Server Transactions respond to those requests with one-or-more responses. The responses may include zero-or-more Provisional (1xx) responses and one-or-more final (2xx-6xx) responses.
Transactions are further categorized as either Invite or Non-Invite. Invite transactions differ in that they can establish a long-running conversation, referred to as a dialog in SIP, and so include an acknowledgment (ACK) of any non-failing final response (e.g., 200 OK).
Because of these transactional mechanisms, SIP can make use of un-reliable transports such as user datagram protocol (UDP).
If we take the above example, User1's UAC uses an invite client transaction to send the initial INVITE (1) message. If no response is received after a timer controlled wait period the UAC may have chosen to terminate the transaction or retransmit the INVITE. However, once a response was received, User1 was confident the INVITE was delivered reliably. User1's UAC then must acknowledge the response. On delivery of the ACK (2) both sides of the transaction are complete. And in this case, a Dialog may have been established.
IM and Presence
The session initiation protocol for instant messaging and presence leveraging extensions (SIMPLE) is the SIP-based suite of standards for instant messaging and presence information. MSRP (message session relay protocol) allows instant message sessions and file transfer.
Many VoIP phone companies allow customers to use their own SIP devices, as SIP-capable telephone sets, or softphones. The market for consumer SIP devices continues to expand, there are many devices such as SIP terminal adapters, SIP gateways, etc.
The free software community started to provide more and more of the SIP technology required to build both end points as well as proxy and registrar servers leading to a commoditization of the technology, which accelerates global adoption. As an example, the open source community at SIPfoundry actively develops a variety of SIP stacks, client applications and SDKs, in addition to entire private branch exchange (IP PBX) solutions that compete in the market against mostly proprietary IP PBX implementations from established vendors.
The National Institute of Standards and Technology (NIST), Advanced Networking Technologies Division provides a public domain implementation of the JAVA Standard for SIP JAIN-SIP which serves as a reference implementation for the standard. The stack can work in proxy server or user agent scenarios and has been used in numerous commercial and research projects. It supports RFC 3261 in full and a number of extension RFCs including RFC 3265.
SIP-enabled video surveillance cameras can make calls to alert the owner or operator that an event has occurred, for example to notify that motion has been detected out-of-hours in a protected area.
Other protocols used in the UC Bridge are H.264 SVC (scalable video coding) is a compression standard that enables video conferencing systems to achieve highly error resilient IP video transmission over the public Internet without quality of service enhanced lines. This standard has enabled wide scale deployment of high definition desktop video conferencing and made possible new architectures which reduce latency between transmitting source and receiver, resulting in fluid communication without pauses.
In addition, an attractive factor for IP videoconferencing is that it is easier to set-up for use with a live videoconferencing call along with Web conferencing for use in data collaboration. These combined technologies enable users to have a much richer multimedia environment for live meetings, collaboration and presentations.
Today, most vendors provide some but not all unified communication products or services and have expertise in different areas of the communications. The result is a fragmented marketplace.